Many 4-cube shelves are built from engineered wood (MDF or particle board) often finished with paper-laminate surfaces. Other common constructions use solid wood frames or powder-coated steel frames.
Internal cubby sizes vary by unit. Example listings show cubbies like 12.5" W x 12.25" D x 10.5" H, and several note compatibility with roughly 13" storage bins (for example, 13" x 13" x 13"). Always check the specific cubby dimensions before buying bins.